Configuration improvements

/etc/rc.conf mechanism was enhanced
Default system settings were moved to /etc/defaults/rc.conf
FreeBSD had prior art on this, and their experience was favourable
The distributed rc.conf reads defaults/rc.conf, and local config settings are added to the end of rc.conf to override the defaults
An optional per-service config override /etc/rc.conf.d/foo, which is read after rc.conf
Useful for automated tools to control service startup
Retains rc.conf semantics - no consensus was reached to remove these